1. Field of the Invention
This invention relates to a method of manufacturing semiconductor devices, and more particularly to a method of processing the surfaces of metal leads of the device so as to suppress corrosion thereof. The invention also relates to a method of selectively depositing metal on prescribed portions of the device.
2. Description of the Prior Art
In a conventional method of manufacturing semiconductor devices, the metal leads of most devices are formed using materials such as Al, an Al-Si alloy or an Al-Si-Cu alloy. The metal leads of such devices are generally formed as follows. Specifically, a thin metal film is formed on a semiconductor substrate by the use of a vacuum evaporation technique, a sputtering technique or the like. Next, a photoresist is painted on the thin metal film so as to form a photoresist film. The photoresist film is then patterned in a prescribed manner. Thereafter, the thin metal film is selectively etched using the thus patterned photoresist film as a mask. This is performed by the use of a dry-etching technique such as an RIE (reactive ion etching) technique or the like. As a result, the thin metal film is formed into metal leads having a prescribed pattern. In general, gas of the Cl group is used for etching metals such as Al or an Al-alloy. However, after such a dry-etching process using the Cl group gas, Cl remains on the surface of the thin film of Al or an AI-alloy. The remaining Cl corrodes the Al film or an AI-alloy film, which is exposed to air without any protection. If one of metal leads is corroded in a semiconductor device, such a metal lead can be broken when the device is operated. This is because the current density in the metal lead becomes abnormally higher. As a result, reliability of the device is significantly lowered.
To prevent the above-described corrosion, the following techniques have been generally employed after the dry-etching process, such as the RIE. Specifically, a technique has been employed in which an electrical discharge is performed using gas of the F group. This causes the surfaces of the metal leads to be fluorinated. Another technique has been employed in which the metal leads are washed with water immediately after the RIE. Another technique has been employed in which heated inert gas is supplied to the metal leads after the RIE. Still another technique has been employed in which a semiconductor substrate to be processed is heated by a heater. The surfaces of metal leads are processed by one of the above-described techniques. Thereafter, the photoresist, which has been used as a mask, is eliminated. Next, the substrate to be processed is further washed with water. In this process, F or Cl still adheres to the surfaces of metal leads. As a result, corrosion of the metal leads inevitably occurs.
FIG. 10 is a schematic perspective view illustrating a part of a conventional semiconductor device. In FIG. 10, a silicon oxide film 102 is formed on the surface of a silicon substrate 101. Next, a metal lead film made of an Al-Si-Cu alloy is formed on the silicon oxide film 102. Thereafter, the metal lead film is selectively etched using a photoresist (not shown) as a mask so as to form metal leads 103. FIG. 10 shows a state in which the photoresist has been eliminated. The substrate 101 of FIG. 10 is washed with high-purity water. In this process, a corroded portion 104 appears at a portion of the metal leads 103 as shown in FIG. 11. This corrosion is caused by F and Cl adhered to the surfaces of metal leads 103. Assume that a semiconductor device is finished leaving the corroded portion 104. When such a device is actually operated, the electric current density in the thinned portion 104 becomes higher. As a result, the portion 104 of metal leads 103 can be easily broken, causing disconnection of the circuit. This significantly lowers the reliability of the semiconductor device.
In the process of eliminating the photoresist, an ashing technique using O.sub.2 gas excitation has been employed. However, in recent years, an ashing technique using gas of the F group has been employed in some cases. In such cases, the metal leads are exposed to gas of the F group, thus F remains on the surfaces of the metal leads. As a result, corrosion occurs more frequently than the case when O.sub.2 gas is used.
Further, such corrosion occurs most significantly in the metal leads of an Al-Si-Cu alloy as compared to those of an Al-Si alloy and to those of Al (least). The reason to use such an alloy is that the electromigration in the metal leads can be significantly suppressed when they are made of an alloy, in particular, of an Al-Si-Cu alloy. Namely, the highest reliability of metal leads can be obtained when an Al-Si-Cu alloy is used. However, metal leads of an alloy have other disadvantages as follows. Specifically, oxidation and reduction can occur in a water solution because of the electrode potential difference between two different metals (i.e., cell effect). For example, in the case of an Al-Si-Cu alloy, corrosion accompanied with the growth of AL(OH).sub.3 occurs when the surfaces of metal leads are exposed to the water solution. This corrosion occurs even when F or Cl does not exist on the metal lead surfaces.
In the process of forming metal leads, a technique in which metal is selectively deposited on metal portions or semiconductors has many advantages. However, only limited numbers of such techniques are available at present. For example, in one technique, W is selectively deposited on W or Si by using WF.sub.6 gas. This technique has advantage as follows. Specifically, the difference in reactive characteristics between a base material and W or Si on which W is deposited can be effectively utilized. However, this technique also has disadvantages as follows. Specifically, this requires a vacuum apparatus. Further, it is difficult to safely handle WF.sub.6 gas. Moreover, this technique is only applicable to W, i.e., no other materials can be used. In another technique, metal is selectivley deposited on prescribed portions of a substrate by using laser beams or electron beams. Specifically, metal is deposited only on prescribed portions to which laser beams or electron beams are applied. This technique is applicable to various metals other than W. The experimental results using this technique can be easily obtained.
For example, literature on this technique, such as D. J Erhlick et al, J.Vac.Sci. & Tech. vol.21, No.1, P.23, 1982, is known. However, this technique requires a large scale system in which laser beams or electron beams are applied to a substrate situated in an atmosphere of gas (or in a solution) containing metal. Therefore, this technique is at present very far from the stage of practical application in view of its higher manufacturing costs and lower productivity.
As described above, in the conventional process of manufacturing semiconductor devices, corrosion of metal leads inevitably occurs, causing reliability of the device to be lowered. Moreover, it is extremely difficult to selectively deposit various metals other than W on the metal and semiconductor portions of the device.